In current art, integrated circuits (ICs) are building blocks for most electronic circuitry. IC technology has grown from single transistor devices to having more than a million circuits in a single IC. Similarly, operating frequencies of microcircuits have grown to 100 megahertz and more. This growth in IC density and speed has benefitted users through development of increasingly smaller, faster, more capable, and more portable electronic devices.
The increased capability, however, comes with an inherent disadvantage in the form of heat. ICs, like most other electrical devices, consume electrical power and dissipate much of the power as heat. Higher circuit densities and higher operating speeds cause a microcircuit to consume greater amounts of power and dissipate greater amounts of heat. It is not uncommon, in current art, for an IC to consume and dissipate 30 to 50 watts.
An inherent disadvantage in current IC technology is that electrical properties of silicon devices change appreciably with temperature changes. IC's function correctly only up to about 200 degrees Celsius (C.) which can be generated by dissipation of about 1 watt of power at room temperature (Integrated Circuit Design and Technology, pg. 51, M. J. Morant, Chapman and Hall, London, 1990). Temperatures over 200 degrees C. can cause an IC to become unstable in its current carrying capability and reduce its reliability and circuit life. IC's are typically packaged in an air-tight environment, such as a plastic-polymer encapsulation, to provide a physically stable environment and to protect sensitive and fragile components.
An inherent disadvantage of plastic-polymer encapsulation of an IC is that it typically decreases an IC's ability to dissipate heat. Because of this disadvantage, methods have been devised to conduct heat away from ICs in encapsulated packages to keep the ICs operating below their maximum stable operating temperatures.
One method known in current art is to replace the die attach pad of a lead frame with a chemically vapor deposited (CVD) diamond substrate, such that the diamond substrate bridges the gap between the IC and the leads in a package. Diamond has a very high thermal conductivity, has outstanding electrical resistivity, and is readily available and cost effective for many applications.
Conventional apparatus and methods for placing diamond substrates in IC packages for heat dissipation is best explained by first describing conventional IC encapsulation apparatus and method. In general, a conventional plastic encapsulation apparatus and method to form IC packages with conductive leads, without a diamond thermal conductor, is as follows: Typically, ICs in die form, also referred in the art as IC die, are attached to mounting areas called islands, or die-attach pads, on metal strips known in the art as lead frames. In this specification, die-attach pad terminology will be used.
Lead frame strips are made of a thin, electrically conductive material and typically have several individual frames with die-attach pads. Each die-attach pad supports an individual IC die during a molding operation. In an IC packaging molding operation, individual dies are encapsulated in a plastic-polymer material, leaving conductive leads protruding from the finished plastic package.
In many cases, densely packaged ICs are manufactured to maximize connectivity by utilizing all four sides of an IC die for connections off-chip. Around the perimeter of each die-attach pad in a lead frame for such die, the frame has a pattern of individual conductive leads for providing electrical connection from wire-bonding pads on the die to outside the package to be formed. The die attach pads and individual leads are formed by selective removal of material in the lead frame, such as by stamping. The number of the leads at a frame with a single die attach pad depends directly on the configuration of a particular IC die to be mounted.
A typical IC die may have over as many as 200 or more external terminations in current art, and each lead frame will have a corresponding number of individual conductive leads. The width of each conductive lead, and separation between adjacent leads, is dependent, among other things, on the size of a finished IC package. The thickness of each conductive lead is the thickness of the lead frame and is predicated on the electrical requirements of the packaged IC during normal operation.
An IC package with external leads for connecting to, for example, a printed circuit board, is typically formed by a plastic-polymer encapsulation process. After IC dies are mounter to die attach pads and thin wires are bonded from each termination pad on the die to the corresponding lead of the lead frame, the lead frame is placed between mating portions of an encapsulation mold, the mold halves are closed, and a liquid-phase polymer is injected to encapsulate IC dies attached to die-attach pads in each frame.
To stop flow of liquid-phase polymer between leads, a lead frame has a pattern of dam bars between individual leads, so a contiguous band of material is formed around the periphery of a die-attach pad. This contiguous dam bar band stops polymer flow, and shapes the outer periphery of an individual package.
After the liquid-phase polymer solidifies and the mold portions are opened, a trimming operation in manufacturing process removes excess plastic in the region around the mold outline and the dam bars as well, leaving the individual leads electrically isolated from one another, and the only thermally conductive path from the IC chip to outside the package other than through the polymer package, is through small wires bonded between the leads and corresponding termination pads on the IC die.
The trimming process is often termed de-junking in the art. The dam bar is removed between each lead, providing electronic integrity for each lead. De-damming is a process of removing all or part of each dam bar by use of a punch with a pattern of teeth conforming to the pattern of dam bars in a lead frame. Typically, de-damming and de-junking can be done in a single step.
In following processing, each lead exposed from the edge of a plastic package may be further treated, such as by cleaning and plating, and the individual IC packages are trimmed from the lead frame strip. Finally, the leads are formed for connectivity to external circuits, such as for Surface Mount Technology (SMT) applications.
In state-of-art manufacturing automated machines are used to perform the encapsulation process. Automated machines are marketed by a number of manufacturers, including several Japanese manufacturers. Such automated machines include molds made to close over one or more lead frames, as described above, whereinafter an encapsulation material, such as liquid-phase polymer, is injected and caused to solidify.
A known apparatus and method for using a CVD diamond substrate in an IC package to conduct heat away from an IC die, adds several steps to the apparatus and methods described above for encapsulating IC packages without a CVD diamond substrate. In general, in this known process, the die attach pads of existing lead frames are entirely removed, or lead frames are produced without die attach pads. Then a diamond substrate is bonded to the leads in place of the die attach pad, and, in effect becomes a new die attach pad.
The diamond substrate in this known process is firmly bonded to the leads, and IC dies are firmly bonded directly to the diamond substrate die attach pad.
After construction of this new lead frame with a diamond substrate die attach pad firmly bonded to the leads, IC dies are bonded to the diamond die attach pads (just as in the previously described process dies are bonded to the metal die attach pads) and the new lead frames with dies attached are substituted in the process already described above for IC packaging without a diamond substrate.
Adding the CVD diamond substrate by replacing the die attach pads in a lead frame entails several new steps in the encapsulation process already described: First, the die attach pad is cut away, a CVD diamond substrate is placed and carefully aligned with the leads, then a mounting operation is performed including curing an adhesive. These extra steps add time and cost to the IC packaging process.
What is clearly needed is an apparatus and method that allows a diamond substrate to be placed in an IC package, using existing manufacturing equipment and processes, without taking extra steps to produce new lead frames or alter existing lead frames to remove die attach pads and to bond diamond substrates to leads of the new lead frame.